The present invention relates to a sphere polisher, and more particularly to a polisher for polishing steel balls for use in a ball bearing.
Hitherto, an apparatus structured as shown in FIGS. 1 and 2 has been known as the above-mentioned polisher. FIG. 1 is a perspective view showing a conventional steel-ball polisher, and FIG. 2 is a perspective view showing an essential portion of the steel-ball polisher shown in FIG. 1.
The steel-ball polisher shown in FIG. 1 has a bed 1 having an end to which a support portion 2 is fixed with bolts. The bed 1 has another end to which a support portion 3 is secured with bolts. The support portion 2 rotatively supports a drive shaft (not shown). A rotative disc 10 made of a grind stone or steel for polishing steel balls is attached to an end of the drive shaft. Note that in the case where the rotative disc 10 is made of steel, a grinding solvent containing a floating abrasive is utilized when the polishing operation is being conducted.
A drive shaft pulley 4 is attached to another end of the drive shaft. The bed 1 includes a drive motor (not shown) so that the rotative disc 10 is rotated by the drive motor through the drive shaft pulley 4.
The support portion 3 movably supports a shaft (not shown) extending coaxially with the drive shaft supported by the support portion 2. The coaxial shaft has an end which faces the support portion 2 and to which a fixed disc 20 is attached. The fixed disc 20 is, by an arbitrary mechanism disposed in the support portion 3, pressed against the rotative disc 10. However, another mechanism is also applicable in which the rotative disc 10 is moved in the axial direction instead of the fixed disc 20 in such a manner that the rotative disc 10 is pressed towards the fixed disc 20.
Thus, the operation for polishing the steel balls by the rotative disc 10 and the fixed disc 20 is adjusted to, for example, three steps consisting of rough machining, medium machining and finishing. On the other hand, a frame 5 is disposed adjacent to another end of the bed 1, the frame 5 having a top surface on which a rotative conveyor 30 is disposed through a pedestal 6.
Referring to FIG. 2, the rotative disc 10, the fixed disc 20 and the rotative conveyor 30 which are essential portions of the steel ball polisher shown in FIG. 1 will now be described.
Referring to FIG. 2, each of the rotative disc 10 which is rotated and the fixed disc 20 disposed coaxially with the rotative disc 10 and arranged to be fixed has a plurality of coaxial annular grooves (concave grooves and complementary grooves) 23 each having a circular arc cross sectional shape, the size of which approximates the curvature radius of the steel balls 22. The annular grooves 23 of the rotative disc 10 and the annular grooves 23 of the fixed disc 20 are formed opposite to each other so that a pair of opposite annular grooves 23 form one polishing circuit for polishing the steel balls 22. The rotative conveyor 30 accommodates the steel balls 22 by a number which is polished at a time and which is administrated as one lot. Between the fixed disc 20 and the rotative conveyor 30, there are disposed a supply chute 24 for supplying the steel balls 22 in the rotative conveyor 30 to the polishing circuit and a discharge chute 25 for discharging, to the rotative conveyor 30, the steel balls 22 polished in the polishing circuit.
The rotative conveyor 30 is formed into a tray like shape having an annular bottom 26a (see FIG. 3) having, in the central portion thereof, a rotation center 26, and an outer frame 27 disposed in the outer periphery of the annular bottom 26a and formed individually from the bottom 26a. A plate-like stopper 29 is secured to the outer frame 27 at a position between the supply chute 24 and the discharge chute 25. Moreover, a steel-ball guide passage 28 is formed by the rotation center 26 and the outer frame 27. The annular bottom 26a of the rotative conveyor 30 is, by an arbitrary rotating mechanism disposed in the pedestal 6, rotated counterclockwise when viewed from a position above the rotative conveyor 30 so as to convey the steel balls 22 accommodated in the rotative conveyor 30 from a position corresponding to the discharge chute 25 of the steel-ball guide passage 28 to a position corresponding to the supply chute 24. Note that the steel balls 22 accommodated in the rotative conveyor 30 are stacked to form a plurality of steps in the rotative conveyor 30.
The operation of the conventional steel-ball polisher having the foregoing structure will now be described.
When the rotative conveyor 30 is operated, the steel balls 22 in the rotative conveyor 30 are allowed to pass through the supply chute 24 so as to be conveyed to the polishing circuit formed by the annular grooves 23 of the rotative disc 10 and the fixed disc 20 forming pairs. The steel balls 22 conveyed to the polishing circuit are, as described above, polished in the polishing circuit by rotating the rotative disc 10 while pressing the fixed disc 20 against the rotative disc 10. The polished steel balls 22 are returned to the rotative conveyor 30 through the common discharge chute 25. The above-mentioned operation is repeated plural times so that the polishing operation is completed.
A ball bearing for use in a HDD (Hard Disk Drive) unit or the like which has been employed as equipment for a computer in recent years must satisfy a severe accuracy in a so-called asynchronous vibration component (Non Repeated Run Out). The non repeated run out is a vibration component which is generated asynchronously with the rotation of the ball bearing. A specific non repeated run out can be reduced by reducing the mutual size differences among the steel balls 22 which are dispersion of the sizes of the steel balls.
However, the conventional steel-ball polisher cannot reduce the mutual size differences (a so called "Lot Diameter Variation" defined by ISO (International Standard Organization) among the steel balls 22 in one lot because of the following reasons.
That is, the steel balls 22 polished in the polishing circuit between the rotative disc 10 and the fixed disc 20 are moved through the common discharge chute 25, the rotative conveyor 30 and the supply chute 24 regardless of the polishing circuit among the plural polishing circuits. Therefore, a random polishing circuit is selected when the steel balls 22 are again introduced into the polishing circuit.
However, the polishing circuits between the rotative disc 10 and the fixed disc 20 have different polishing conditions between the outer polishing circuits and the inner polishing circuits in the direction of the radius of the rotative disc 10 and the fixed disc 20 because of the differences in the polishing distance, the peripheral velocity, the amount of the applied polishing solution, the number of flowing steel balls and the like.
Therefore, if the steel balls 22 are allowed to randomly pass through the concentric polishing circuits having different polishing conditions between the inner polishing circuits and the outer polishing circuits in the radial direction, the diameters of the steel balls 22 are varied due to the difference in the polishing circuits through which the steel balls 22 have been allowed to pass through. Therefore, steel balls 22 even included in the same lot have different diameters.
Moreover, the steel balls 22 accommodated in the rotative conveyor 30 in a stacked state have a problem in that the flow of the steel balls 22 becomes non-uniform in the steel-ball guide passage 28 because of frictional resistance to be described later.
Referring to FIG. 3, the frictional resistance and the operations of the steel balls 22 in the steel-ball guide passage 28 will now be described. FIG. 3 is a diagram showing the frictional resistance applied to the steel balls 22 and the operations of the steel balls 22 in the steel-ball guide passage 28.
The outer frame 27 of the rotative conveyor 30 is formed by a metal plate or in the form in which a cushioning member, such as a rubber sheet, is applied to an inner guide surface 27a of the outer frame 27 to protect the steel balls 22 from being damaged. Therefore, a line of the steel balls 22 which are brought into contact with the inner guide surface 27a of the outer frame 27 is applied with great frictional resistance (point a shown in FIG. 3) from the inner guide surface 27a of the outer frame 27. Since the foregoing frictional resistance is considerably greater than slide resistance (point b shown in FIG. 3) with the other steel balls 22, the steel balls 22 which are brought into contact with the inner guide surface 27a of the outer frame 27 rotate. Moreover, the speed of the steel balls 22 passing through the steel-ball guide passage 28 is reduced as compared with the moving speed of the bottom 26a of the rotative conveyor 30. On the other hand, the steel balls 22 which are not brought into contact with the inner guide surface 27a of the outer frame 27 are moved at substantially the same speed as the moving speed of the annular bottom 26a of the rotative conveyor 30.
As described above, the steel balls 22, which are brought into contact with the inner guide surface 27a of the outer frame 27 are conveyed slowly as compared with the other steel balls 22. Therefore, the number of passes through the polishing circuits becomes different. Moreover, the surfaces of the steel balls 22 are damaged attributable to sliding with the other steel balls 22. Thus, the state of surface finishing of the steel balls 22 are adversely affected.
In addition, in the case where the rotation speed of the rotative conveyor 30 is increased so as to make an efficiency higher, some of the steel balls 22 possibly interrupt the flow defined by the another steel balls 22. As a result of this, some of the steel balls may not be supplied or fed into some of polishing circuits. Therefore, this approach not only makes the efficiency as a whole lower but also makes the numbers of steel balls which are being simultaneously polished lower. Further, this approach is one of causes to increase of the lot diameter variation in the same lot.